It is well known that, prior to the manufacture and use of electronic equipment today, shielding materials should be installed or otherwise employed to control and/or minimize electronic noise, such as electromagnetic interference (EMI) or radio frequency interference (RFI), emanating from the electronic equipment itself, from power sources, or from wires interconnecting the equipment to the power sources, as well as from the ingress of electronic noises from outside sources.
Installation of appropriate shielding materials can be undertaken at the time of original construction. For example, U.S. Pat. No. 3,322,879 to LINDGREN discloses a soundproof prefabricated demountable panel wall unit having two spaced electrically isolated layers of sheet metal shielding, while U.S. Pat. No. 2,961,478 to BURNS discloses an enclosure of the type defining a "free-space" room in which the walls are not only thermally insulated, but also shielded against electromagnetic and radio frequency interference.
On the other hand, shielding materials may be installed as a retrofit procedure. For example, U.S. Pat. No. 3,517,627 to KRUSE, U.S. Pat. No. 4,733,013 to HEMMING, and U.S. Pat. No. 4,647,714 to GOTO disclose protective shielding material spacially mounted atop the exterior surface of an existing wall, floor or ceiling, for minimizing or eliminating noise of the type caused by electromagnetic or radio frequency interference. GOTO teaches applying protective shielding material in the form of a metal-containing laminated sheet applied atop an existing wall surface as wall paper.
In the past, the material of choice for EMI and RFI shielding materials was predominantly sheet metal. More recently, however, metallic foils and laminates of metal foil bonded to reinforcing substrates are being used with greater effectiveness (see IEEE Electrical Insulation Magazine, January/February 1988, Vol. 4, No. 1, pp 11-13).
The problems of greatest concern in the installation of any type of flexible shielding material relate to the manner in which edge regions of adjacent panels have been joined together. U.S. Pat. No. 4,733,013 to HEMMING discloses a joint and a method of its formation wherein two panels of shielding material are sandwiched between two strips of shielding material, and the entire sandwich assembly is connected, via threaded fasteners, to an electrically insulating member.
Recently, it has been found that the difficulties encountered with the use of rigid sheet material differ from those encountered with flexible materials, including both metallic foils, and foil laminates. For example, optimization of the effectiveness of the flexible shielding materials depends, to a great extent, on the continuity of electrical contact between overlapping portions of the flexible material. Continuity of electrical contact at joints affects the ability of the shielding material to eliminate undesired electromagnetic and radio frequency signals, and is diminished when adjacent portions of flexible metallic shielding material sustain relative movement. There is presently no known method for joining edge or overlying regions of flexible shielding materials such that the connection of the overlying regions exhibits optimum electrical continuity, and thus optimum EMI and RFI shielding characteristics.